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ISON Image of the Week

The CIOC's Chair, Casey Lisse, was the lead on an investigation using the Chandra X-Ray Telescope to observe comet ISON. Only in the past 20-years have we known that comets emit X-rays, so it's a great result to see ISON shining brightly at these wavelengths! [Credit: Casey Lisse/Chandra]

When asked why we study comets, one of our favorite analogies to use is that comets are like the dinosaur bones of the solar system. They are an ancient relic of what once was, and yield vital clues that tell us how our solar system was pieced together. By looking at the composition and structure of comets we can begin to get a feel for how the early universe material began to clump together and form the planets and the Sun.

Unfortunately, studying the composition of comets is difficult as we can't often send probes physically to comets (though ESA's Rosetta is soon to do just that!), meaning we have to get a little more creative in our methods. One of the tricks we have up our sleeve is to look at both the visible and invisible parts of electromagnetic spectrum that are emitted or absorbed by the comet.

Historically, astronomers were limited to just visible light that they could see with their own eyes. But over time telescope filters were created and, eventually, we developed the capability to view astronomical objects from outside of the Earth's atmosphere. The advent of space-based astronomy heralded a revolution in astronomy that enabled views of the Sun, stars and distant galaxies in wavelengths of radiation previously invisible to us, such as X-rays.

As soon as we had this new capability, astronomers began launching X-ray detectors into space and capturing new views of our solar system, galaxy and universe. But of all the sources of X-rays we have in our solar system, perhaps one of the most surprising is comets!

In 1996, the CIOC's Chair, Casey Lisse, led a study that first discovered x-ray emission in comets, when he and x-ray astronomers Konrad Dennerl, Jakob Englehauser, Joachim Truemper, and John Pye detected x-ray emission from Comet Hyakutake via the German ROSAT satellite. Through subsequent studies using ROSAT, EUVE, Chandra, and XMM, it has been shown that the emission comes from the "charge exchange" between neutral atoms and molecules in the comet's coma and highly ionized O, C, N, Ne, Si, and Mg ions in the solar wind that is streaming by the comet. This process has now been observed in over 25 comets, and it is through studying this interaction that we can build a picture of the abundances of particles in both the comet's coma and in the solar wind flowing over it.

And now we are delighted to say that a new observing program led again by Casey Lisse has successfully detected comet ISON using the NASA Chandra X-ray observatory! The image above shows the comet as detected in the ACIS-S low resolution spectro-photometer , which detects 0.3 - 2.0 keV X-Ray photons. Casey comments that comet ISON looks "very bright for a 2x1028 molecules/sec comet. The morphology looks pretty normal, with maximum brightness closest to the nucleus, but a bit offset towards the Sun.". The 2x1028 mol/sec refers to the rate at which ISON is producing water - a value that has been determined from other observations, and corresponds to around 16 Olympic-sized swimming pools of water every day! It's about average, if a little on the low side, for a typical comet.

Chandra is now the ninth spacecraft to have obtained comet ISON images, and it's Casey's plan to use the telescope again to observe ISON after perihelion - assuming the comet survives of course! His goal is to obtain data that allows a comparison of the comet both pre- and post-perihelion, with the results hopefully giving us valuable information about the processes that occurred on the comet during this most extreme part of its orbit.

We are now less than three weeks from perihelion, and in the most high-risk phase of comet ISON's orbit. If it is going to fall apart on us, and fragment or fizzle, then the next couple of weeks are when it will most likely happen. We truly have no idea of whether this will or will not happen, and whether the comet will reach or survive perihelion, but stay tuned to the CIOC website and we will certainly keep you up-to-date on ISON's latest status.
Every week this year we will put up a new image related to Comet C/2012 S1 (ISON). If you have a cool image you'd like us to consider, please send it to sungrazer@nrl.navy.mil, along with a description and any credits you would want applied. We'll contact you if we choose to use your image on the CIOC Website.

Blog archive

Image of the Week

Every week, a new image related to Comet ISON!

This week: The Final Nail in the Coffin? When ground-based observers came up empty-handed in the hunt for ISON's remains, we turned to the Hubble Telescope to see if its ultra-powerful instruments could find anything...Earlier picks can be found in the ISON Image of the Week Archives. Note that this feature will become monthly, instead of weekly, in 2014.